Light signals are required for the induction and regulation of many developmental processes in plants. The purpose of the proposal is to determine the primary event in photoreceptor signaling and to begin to unravel the control circuits of light-regulated development in plants. Though previous physiological and molecular studies have revealed the variety and complexity of plant responses to light, they have not answered questions about the mechanisms of those responses. Genetic analysis demonstrates that light responses are not simply endpoints of linear signal transduction pathways, but are the result of the integration of a variety of input signals through a complex network of interacting signaling components. As with other signal transduction systems, it is unclear whether relatively few signaling components regulate a multitude of responses, or whether the variety of responses correlates with a high number of specific regulatory molecules. Molecular biology and genetics have led to the identification of two main classes of regulatory components: Photoreceptor genes, encoding either red/far-red light receptors, phytochromes, or a putative blue-light receptor, and genes that act far downstream in the signal transduction pathway from the photoreceptor, presumably in the nucleus. It is proposed to use biochemical and genetic strategies to identify and characterize signal transduction components that act early in a phytochrome signal transduction pathway. The primary aims of these studies are (1) to identify proteins that interact directly with phytochrome B and to determine if these protein(s) interact solely with phytochrome B or also interact with other phytochromes; (2) to use genetics to identify components of signaling pathways that are specific to this particular phytochrome; (3) to clone at least one gene acting in a phytochrome B-specific pathway; and (4) to identify mutations in additional phytochromes. To identify proteins that interact directly with phytochrome B, we have used cloned PHYB sequences to identify genes from an Arabidopsis expression library that encode proteins that interact with the C-terminus of phytochrome B. The specificity of this interaction both in vivo and in vitro will be determined. To identify signaling components acting specifically downstream of the light-stable phytochrome B, but not downstream of the light-labile phytochrome A, we will identify and characterize genes whose activity is required for the expression of the phyB phenotype. In summary, the proposed experiments will further our knowledge of phytochrome-mediated signal transduction pathways, and will be a step toward elucidating how information is transmitted from photoreceptors in the cytoplasm to regulatory factors in the nucleus. The long-term objective is to completely understand the signal transduction network that allows plants to perceive and respond to their light environment, and to regulate this to general mechanisms of signal transduction in other organisms.